The present invention relates to a ferrofluid composition having improved oxidation resistance and a method for increasing the gelation time of a ferrofluid.
Super paramagnetic fluids, commonly referred to as ferrofluids, are colloidal suspensions of magnetic particles suspended in a carrier liquid. The magnetic particles are suspended in the carrier liquid by a dispersing agent which attaches to the surface of the magnetic particles to physically separate the particles from each other. Dispersing agents, or dispersants, are molecules which have a polar "head" or anchor group which attaches to the magnetic particle and a "tail" which extends outwardly from the particle surface.
Magnetic fluids have a wide variety of industrial and scientific applications which are known to those skilled in the art. Magnetic fluids can be positioned and held in space, without a container, by a magnetic field. This unique property has led to the use of magnetic fluids as liquid seals which have low drag torque and which do not generate particles during dynamic operation, as conventional lip seals tend to do. Specific uses of magnetic fluids which illustrate the present invention and its advantages include the use of magnetic liquids as components of exclusion seals for computer disk drives, seals and lubricants for bearings, for pressure and vacuum sealing devices, for heat transfer and damping fluids in audio speaker devices and for inertia damping.
In many sealing applications which use a magnetic colloid sealing system, it is particularly advantageous to have a magnetic colloid with the lowest possible viscosity to reduce frictional heating. This, in turn, reduces the temperature of the fluid in the seal and consequently the evaporation rate of the carrier liquid, thereby prolonging the life of the seal. Ideally, magnetic fluids suitable for sealing disk drives for computers have both a low viscosity and a low evaporation rate.
These two physical characteristics of magnetic fluids are primarily determined by the physical and chemical characteristics of the carrier liquid. According to the Einstein relationship, the viscosity of an ideal colloid is: EQU (N/N.sub.0)=1+.alpha..PHI.
wherein
N is the colloid viscosity; PA1 N.sub.0 is the carrier liquid viscosity; PA1 .alpha. is a constant; and PA1 .PHI.is the disperse phase volume.
The saturation magnetization (G) of magnetic fluids is a function of the disperse phase volume of magnetic material in the magnetic fluid. In magnetic fluids the actual disperse phase volume is equal to the phase volume of magnetic particles plus the phase volume of the attached dispersant.
The ideal saturation magnetization for a magnetic fluid is determined by the application or the magnetic design.
The higher the magnetic particle content, the higher the saturation magnetization. Also, a set volume % of metal particles in the fluid such as cobalt and iron generates a higher saturation magnetization than the same volume % of ferrite. In other words, saturation magnetization of a fluid is determined by the amount and type of magnetic particles in the fluid.
By way of example, typical saturation magnetization values for: (1) exclusion seals for a hard disk drive are between 200-300, preferably 250 G; (2) vacuum seals for the semiconductor industry are between 300-600, preferably 450; (3) loud speakers are between 100-400, preferably 100; and (4) fluid film bearings for a motor and tester are between 50-300.
Magnetic particle size and size distribution, along with the physical and chemical characteristics of the dispersant, also affect the viscosity and, consequently, the evaporation rate of magnetic fluids.
There are, however, a number of ways that a ferrofluid can lose its effectiveness, such as evaporation of the carrier liquid. Oxidative degradation, which occurs when the fluid is heated in the presence of air, is another problem.
Oxidative degradation of the magnetic particles causes the particles to lose their magnetic character due to the formation on the surface of the particles of a non-magnetic or low magnetic oxide layer. Attempts to solve this problem, i.e., prevent oxidation of the magnetic particles, are described in U.S. Pat. Nos. 4,608,186, 4,624,797 and 4,626,370.
In addition to oxidative degradation of the magnetic particles, oxidative degradation of the dispersant is believed to be another problem associated with the loss of effectiveness of a ferrofluid. Oxidative degradation of the dispersant increases the particle-to-particle attraction within the colloid, resulting in gelation of the magnetic colloid at a much more rapid rate than would occur in the absence of oxidative degradation. Accordingly, there is a need in the art for a ferrofluid having an improved resistance to oxidative degradation of the dispersant to increase the time until gelation occurs.